Unwanted material thrown overboard (6)
I believe the answer is:
jetsam
'unwanted material thrown overboard' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for jetsam that I've seen before include "Goods washed ashore from shipwreck" , "Discarded rubbish" , "lost cargo" , "Rubbish on shore thrown from ships" , "Floating rubbish" .)